1. Field of the Invention
The present invention relates to an optical disc apparatus and, more particularly, to an apparatus for reproducing signals or data recorded on a compact disc, video disc or the like.
2. Description of the Prior Art
In an optical disc player, it is generally customary that a light beam is positioned by cooperation of a rough actuator which comprises a linear motor and so forth for driving a pickup-base, and a fine actuator for driving a focus lens mounted on the pickupbase.
For example, when an access is executed in the ordinary optical disc player, first the pickup-base is positioned approximately at a desired target point by the rough actuator, and then the fine actuator mounted on the pickup-base is driven under fine control to position the focus lens exactly on the target track.
However, in case the rough actuator is driven at a high speed for a fast access, the focus lens held by the fine actuator is deflected due to the acceleration caused at the start or halt of the rough actuator or by the vibration induced during the travel thereof, so that the fine actuator is kept in vibration for a while even after the halt of the pickup-base in the vicinity of the target, thereby raising a problem that the fine actuator cannot be brought into servo control with stability and rapidity.
In an attempt to solve the above problem, there is proposed an improved system wherein a position detector is disposed in the vicinity of the fine actuator, and a position servo loop applying the output signal of the position detector as an error signal is formed during the travel of the pickup-base to lock the fine actuator at its mechanically neutral point, thereby preventing vibration of the fine actuator.
However, there still remain some problems unsolved with regard to complication in the structure of the position detector and difficulties in constituting a stable position servo circuit due to the d-c drift in the component elements of the position servo circuit, and also the secular change caused therein such as temperature fluctuation in particular.
For the purpose of eliminating the above-described drawbacks existing in forming a stable position servo circuit, an improvement has been accomplished as disclosed in Japanese Patent Laid-open No. 61 (1986)-214230, wherein a position servo loop is so formed that the position of the fine actuator immediately before a fast displacement of the pickup-base is stored in a hold circuit and the fine actuator is retained at such position during the travel of the pickup-base.
According to the above system, a position detector is disposed in the vicinity of the fine actuator to produce an output serving as a position error signal, and a position servo loop is formed in such a manner as to use, as a reference servo signal, the position error signal obtained from the position detector and stored in the hold circuit immediately before the fast displacement of the pickup-base, so that despite any variation in the characteristic of the position detector, the servo control is effected while such variation is absorbed to consequently settle the problems including the aforementioned d-c drift, secular change and temperature fluctuation.
In the above system, however, it becomes necessary to employ a hold circuit for storing the position data therein, hence failing in solution of the problem with regard to the complicated structure of the apparatus.
Furthermore, according to the above system, a reference position signal of the position servo loop is derived from the position error signal obtained by holding the position error output, which is produced from the position detector disposed in the vicinity of the fine actuator, at the value immediately before start of the travel of the pickup-base. Therefore, although it may be possible to solve the problem of vibration of the pickup-base during its travel, another disadvantage is still existent that, if the pickup-base begins its travel with the fine actuator inclined due to some eccentricity of the disc or the like, the fine actuator is driven continuously while being retained in such inclined posture. And when the travel of the pickup-base is brought to a halt, the fine actuator is still kept in the inclined posture and the tracking servo mode is selected in such state, so that the operation is limited by the dynamic range of the servo circuit and the mechanical dynamic range of the fine actuator to eventually bring about instability with respect to the tracking servo pull-in action.
Moreover, upon occurrence of any dropout during reproduction of signals from a disc (or playing of a disc) in the conventional optical disc player, a great disturbance signal is generated in each of the error signals in the focus servo loop and the tracking servo loop formed to control the fine actuator which supports the focus lens therein, hence causing a trouble that the focus lens is positionally deviated.
In an attempt to solve this problem, an improved invention has been contrived as described in Japanese Patent Laid-open No. 59 (1984)-38980. According to the technique disclosed, in reproduction of signals from any disc having many dropouts, the gain of each servo loop is lowered by means of a manual switch to relatively reduce the amplitude of the disturbance signal generated during the occurrence of such dropouts, thereby minimizing the deviation of the focus lens.
But an important problem is still left unsolved in the system mentioned above that, when setting a stable servo gain for each loop, it becomes necessary to previously find a disc having many dropouts (as the amount of dropouts is unknown until signals recorded on the disc is actually reproduced in the player) and, even with prior setting of an optimal gain by reduction of its value, there occurs an adverse side effect that the operation is influenced readily by external mechanical vibration.
There is further known another system with regard to a countermeasure for such dropout, wherein, during generation of the dropout detected by some other means, the servo loop is opened so that the disturbance signal is not transmitted to the fine actuator, thereby preventing a positional deviation of the focus lens.
The system mentioned above is capable of relatively diminishing the adverse side effect that the operation is influenced by the external vibration, since the servo loop is opened merely for the moment of the dropout.
However, because of the structure where the focus lens is supported by a mechanical damper, the restoring force derived from the elasticity of the damper itself is exerted to move the focus lens which is placed in a noncontrolled state during generation of the dropout. Consequently, at the termination of the dropout, the focus lens thus moved is on another track spaced apart from the desired former track prior to the dropout.
Furthermore, in some of the laser disc players manufactured in a relatively early stage of the technical development, the focus lens is not furnished with a mechanical damper in regard to the focus controlling direction and is supported merely by electromagnetic means (focus servo) alone, so that the position of the focus lens is varied due to the gravity during the dropout period and a great servo error signal corresponding to such positional deviation is generated at the termination of the dropout to eventually bring about instability of the operation.